The invention relates to thermal recording apparatus, and more particularly, to a thermal recording apparatus utilizing a plurality of aligned thermal resistive elements selectively driven according to recording signals.
In the prior art thermal recording apparatus used in facsimile systems a matrix circuit has been used, for example, to decrease the number of drive elements and lead lines connected to the resistive elements. Toshiba has recently introduced one method which eliminates the matrix circuiting by incorporating a shift register; such a method produces higher recording speed. A structure embodying this method is shown in FIG. 1. Serial binary recording signals (picture signals) for one line and received from input 1 are stored in a shift register 2 having memory units corresponding to the number of thermal resistive elements 4. The signals stored in register 2 are read out, in parallel, and supplied to drive elements 3. Drive elements 3 comprise, for example, transistor switching elements and are connected respectively to thermal resistive elements 4. As a result, current is supplied from a DC power source 5 to the thermal resistive elements 4 corresponding to the particular switching elements which are energized by the signals supplied from shift register 2. When binary recording signals are supplied to input terminal 1 and thermal sensitive paper (not shown), contacting the resistive elements 4, is moved with respect to elements 4, a recorded image is thereby obtained on the paper.
In this apparatus, the current cpacity of DC power source 5 is determined by considering the case when all the recording signals for one line are black signals (i.e., printing signals). In that event, the current output of source 5 will be at its maximum level. For example, if recording is executed with a resolution of 8 dot/mm on a thermal sensitive paper (e.g., size A4), the required number of thermal resistive elements which must be employed in 1728. If the voltage of DC power source 5 is 18 volts and the resistance of thermal resistive element 5 is 300.OMEGA., the current flowing through each thermal resistive element is 60 mA. Therefore, when all the recording signals (i.e., 1728 bits) are black signals, the total current output of source 5 becomes 103.68 amperes. Consequently, a DC power source having a large current capcity is necessary.
While the probability that all the recording signals for one line will be black signals is low, it is still necessary to have that capability if the need should arise in order to ensure proper print quality. Using a power source which has a large current capacity, however, results in economic waste with the size of the apparatus being concomitantly increased. One method to solve this problem is disclosed in Japanese patent disclosure No. 53-13434. This system counts the number of black recording signals for one line by utilizing a counter. When the number of black signals is larger than the current capacity of the power source, certain groups of the thermal resistive elements which had been simultaneously driven are de-energized. This technique, however, uses a matrix circuit for supplying recording signals to thyristor elements serially connected to the thermal resistive elements. As a result, the recording speed is reduced. Further, each time the number of black signals is larger than the current capacity, the recording of a line must be executed by sequentially de-energizing certain elements while sequentially energizing different elements. Since switching is needed each time the number of black signals is less than the current capacity, the demand for such sequential switching is frequent; therefore, more demands are placed on the switching circuitry.